Mixing circuit comprising a self-oscillating triode with intermediate-frequency feed-back



March 25, 1958 MIXIN TRIOD Filed Feb.

INVEN TOR JACOBUS JOHANNES RONGEN AGENT United States PateIItU MIXING CIRCUIT COMPRISING A SELF-OSCIL- LATING TRIODE WITH INTERMEDIATE-FRE- QUENCY FEEDBACK Jacobus Johannes Rongen, Eindhoven, Netherlands, as-

signor, by mesne assignments, to North American Philips-Company, Inc., New York,-N.-Y., a corporation of'Delaware Application February 5, 1954-,Serial No. 408,512

Claims priority, application Netherlands 1 February 12, 1953 4 Claims. (Cl. 250-20) The invention relates to' a mixing circuit comprising a self-oscillating triode with" feed-back of the intermediateconstituted by a triode used in a self-oscillating circuit. 25

The reason is that the signal-noise ratio of such a tube is low and that the arrangement is simple and hence cheap.

This, however, has a limitation in that in receivers having a comparatively narrow frequency band for example frequency-modulation receivers, a material damping is exerted on the bandpass filter following the mixing tube, since the internal resistance of the triode is low. As a consequence the selectivity is bad and the mixer amplification cannot attain its optimum value.

It has been suggested to feed part of the intermediatefrequency voltage at the anode of the triode in phase opposition back to the grid. The tube has thus produced in it a current in phase opposition to the damping current in the tube due to the internal conductivity. Thus a par- 4 tial or complete compensation may be obtained. Even over-compensation is possible.

The compensation arrangement may furthermore be such that for the grid of the oscillator tube a low-frequency negative feed-back is obtained. Thus the value of the leak resistance becomes materially higher than in the absence of this negative feed-back. Since the noise of the mixing tube is lower according as the grid current is lower, the use of a high leak resistance provides an appreciable improvement in the signal-noise ratio 50 of the mixing stage.

In the methods of compensating the internal resistance hitherto carried out the intermediate-frequency voltage to be fed back was supplied to the side of the preceding high-frequency circuit remote from the grid.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing.

Fig. 1 shows a circuit arrangement in accordance with the prior art. Herein reference numeral 1 designates the mixing triode, 2 the high-frequency input circuit, to which the incoming signal is supplied and 3 the circuit determining the oscillator frequency. The inductance of the latter is coupled inductively with an inductor 4, which is connected in series with a capacitor 5 between the grid and the cathode of tube 1. The end of the circuit 2, not connected to earth is connected to a tapping, in general the centre, of the inductor 4. On the one hand the circuit 3 is connected to earth and on the other hand through a capacitor to the anode. In this circuit are thus produced the oscillations of the oscillator, which produce the intermediate-frequency oscillations when mixed together with the incoming oscillations, these inintermediate-frequency oscillations being transmitted to the circuit 7 through the coil 6 included in the anode cir- 5 -cuit, this circuit '7 being' tuned to the intermediate frequency. -Theintermediate=frequency oscillations are taken -from" this circuit for further amplification and demodulation. it

The'inductoris'connected through a resistor 9 to the 10 po'sitive'ter'minalof thesupply source. From the junction of th e' inductor-6 andtheresistor 9 is derived an intermediate-frequency-voltage, which is fed-to the junction of the inductor of the circuit 2 and a capacitor connected in 'se'ries therewith. Thus a negatively damping feed hack-is obtainedfor "the intermediate-frequencyoscillations.

The circuit arrangement-described" above has the disadvantage thatowing to the feed-back of the intermediatefr'equency voltage to the'lowerside' of the high-frequency circuit the degree of compensation of the internal resistance of the triode varies with the position of the tuning capacitor of the high-frequency circuit 2, since in most cases it is desirable or necessary to connect the latter on one side to earth. A further disadvantage is that during manufacture it is more difiicult to reproduce the highfrequency circuit with respect to the tolerances in component parts and the mounting. The connecting wires will always exhibit a certain degree of inductance, which exerts an influence on high-frequencies, which cannot be neglected.

The invention has for its object to provide an improvement therein. It consists in that the primary inductor of the intermediate-frequency bandpass filter 6, 7 is connected electrically directly between the grid and the anode of the tube. In this case measures must, of course, be

taken to cut off the direct anode voltage. This may be carried out by feeding the fed-back intermediate-frequency voltage to the junction of the series combination of two capacitors included in the grid circuit.

Figs. 2 to 7 show a few embodiments of such a circuit arrangement. The various parts are designated by the same reference numerals as the corresponding parts of the arrangement shown in Fig. 1.

In the circuit arrangement shown in Fig. 2 two capacitors 10 and 11 are included between the coil 4 and the grid of the tube 1, the junction of these capacitors being connected to the junction of the coil 6 and the resistor 9. The degree of compensation is determined by the capacities, among which are included the output capacity and the input capacity of the tube. In this case the capacity of the capacitor between the circuit 3 and the anode and that of the capacitor 5 play a principal part. Both the high-frequency coil and the oscillator coil constitute sub- 5 stantially a short-circuit for the intermediate-frequency signal.

Fig. 3 shows a slightly modified arrangement. In this case the fed-back intermediate-frequency voltage is supplied to the tapping of the coil 4. In this case also the capacity of the capacitor 5, for neutralizing the gridcathode capacity plays a part.

In the arrangement shown in Fig. 4 a delta-connection of the oscillator circuit is employed. The tapping of the oscillator coil must be considered to be connected to earth as far as the intermediate frequency is concerned.

Fig. 5 shows an oscillator arrangement comprising a capacitative cathode impedance. The grid circuit of the oscillator comprises two relatively balanced parts 3 and 12, so that the high-frequency oscillation may be coupled with the oscillator circuit in a simple manner. As in the arrangement shown in Fig. 3 the intermediate-frequency tions to an electrode of l. A mixing circuit arrangement comprising a triode tube having a grid and an anode, an oscillatory circuit connected to an electrode of said triode to cause selfoscillations, at source of input oscillations, signal-conducting means comprising two capacitors connected in series to provide a path for feeding said input oscillasaid tube whereby said input oscillations and said self-oscillations are mixed to provide intermediate-frequency oscillations, and an intermediatefrequency bandpass filter having a primary inductance directly connected electrically between said anode and the junction of said two series-connected capacitors thereby providing feedback of said intermediate-frequency oscillations.

2. A mixing circuit as claimed in claim 1, in which said signal-conducting means further comprises a feedback coil connected between said series capacitors and said source of input oscillations and coupled to said oscillatory circuit.

3. A mixing circuit as claimed in claim 1, in which said signal-conducting means further comprises a feedback coil connected between said series capacitors and said grid and coupled to said oscillatory circuit.

4. A mixing circuit as claimed in claim 1, including a source of operating voltage and a resistor connected at an end thereof to a terminal of said source of operating voltage, the remaining end of said resistor being connected to the end of said primary inductance which is connected electrically to said junction of the two series-connected capacitors.

References Cited in the file of this patent UNITED STATES PATENTS 

